1. Field of the Invention
The present invention relates to a perovskite manganese oxide thin film. More specifically, the present invention relates to a perovskite manganese oxide thin film, the electrical, magnetic or optical properties of which are switched in response to a stimulus such as temperature, electrical field, magnetic field or light exposure.
2. Background of the Related Art
There has been concern in recent years that semiconductor devices may be facing the limits of the scaling law, which has been a guiding principle of performance advances in the field. In this context, materials are being developed to enable new operating principles in order to weather the crisis when the transistor limit is reached. For example, in the field of spintronics, which exploits the spin degrees of freedom of electrons, development has been aimed at high-density non-volatile memories capable of high-speed operation at the same level as DRAM (dynamic random access memory).
There has also been progress in research into materials having strongly-correlated electron systems that cannot be described in terms of band theory, which is a cornerstone of semiconductor device design. Substances have been discovered that exhibit very large and rapid physical changes caused by phase transitions in the electron system. In strongly correlated electron system materials, a variety of electron phases with a variety of orders formed by spins, charges and orbitals are possible because the phase state of the electron system is affected not only by the spin degrees of freedom but also by the degrees of freedom of the electron orbitals. Typical examples of strongly correlated electron system materials are the perovskite manganese oxides, in which a first order phase transition produces a charge-ordered phase by alignment of 3d electrons of manganese (Mn) or an orbital-ordered phase by alignment of the electron orbitals.
In a charge-ordered phase or orbital-ordered phase, electrical resistance increases because the carrier is localized, and the electron phase becomes an insulator phase. The magnetic behavior of this electron phase is that of an antiferromagnetic phase due to the double exchange interactions. The electron states of the charge-ordered phase and orbital-ordered phase should often be regarded as semiconductor states. This is because although the carrier is localized in the charge-ordered phase or orbital-ordered phase, the electrical resistance is lower than that of a so-called band insulator. In accordance with convention, however, the electron phase of the charge-ordered phase or orbital-ordered phase is here called an insulator phase. Conversely, when the behavior is metallic with low resistance, the electron phase is a ferromagnetic phase because the spins are aligned. The term “metallic phase” is defined in various ways, but in the present application a metallic phase is one in which “the temperature derivative of resistivity is positively signed”. Expressed in this way, the aforementioned insulator phase can be re-defined as one in which “the temperature derivative of resistivity is negative”.
A variety of switching phenomena have reportedly been observed in bulk single-crystal materials made of substances capable of assuming either the aforementioned charge-ordered phase or orbital-ordered phase, or a phase that combines both a charge-ordered phase and an orbital-ordered phase (charge- and orbital-ordered phase) (Patent Document 1: Japanese Patent Application Publication No. H8-133894; Patent Document 2: Japanese Patent Application Publication No. H10-255481; and Patent Document 3: Japanese Patent Application Publication No. H10-261291). These switching phenomena occur in response to applied stimuli, namely, temperature changes around the transition point, application of a magnetic or electric field, or light exposure. These switching phenomena are typically observed as very large changes in electrical resistance and antiferromagnetic-ferromagnetic phase transitions. For example, resistance changes by orders of magnitude in response to application of a magnetic field are a well-known phenomenon called colossal magnetoresistance.
To achieve any device with a high degree of utility using a perovskite manganese oxide, these switching phenomena must be manifested at room temperature or above, such as an absolute temperature of 300 K or more. However, the switching phenomena disclosed in the aforementioned documents have all been verified only under low-temperature conditions of about liquid nitrogen temperature (77 K) or less for example. In the perovskite manganese oxides disclosed in the aforementioned documents, trivalent rare earth cations (hereunder represented as “Ln”) and a divalent alkaline-earth (“Ae”) randomly occupy the A sites in the perovskite crystal structure, and it is thought that the temperature at which the switching phenomena are manifested is lowered as a result of this randomness. It is known Second Substitute Specification (U.S. Nat. Stage of PCT/JP2012/055344) that the transition temperature for the charge-ordered phase can be elevated to about 500 K by ordering the A-site ions in an AeO-BO2-LnO-BO2-AeO-BO2-LnO-BO2 . . . configuration. Regular arrangement of the ions occupying the A sites as in this example is called “A-site ordering” below. A feature of the group of substances exhibiting such high transition temperatures is that they contain Ba (barium) as an alkaline-earth Ae. For example, transition temperatures above room temperature have been reported with substances containing Ba as an alkaline-earth Ae, and using Y (yttrium), Ho (holmium), Dy (dysprosium), Tb (terbium), Gd (gadolinium), Eu (europium) and Sm (samarium), which have small ionic radii, as a rare earth Ln.
For these switching phenomena to be applied to an electronic device, magnetic device or optical device, they must be manifested when the oxide has been formed as a thin film. Conventionally, the problem has been that even if a single crystal of perovskite manganese oxide is formed as a thin film on a (100) oriented substrate, the switching phenomena are not manifested in the resulting (100) oriented perovskite manganese oxide single-crystal thin film. This is due to suppression of a type of lattice deformation called Jahn-Teller deformation, which is required for the first order phase transition to a charge-ordered phase or orbital-ordered phase. This is due to the fact that the in-plane crystal lattice of the single-crystal thin film is fixed to the crystal lattice of the substrate in the plane of the substrate, and exhibits fourfold symmetry in the substrate plane.
On the other hand, Patent Document 4 (Japanese Patent Application Publication No. 2005-213078) discloses forming a perovskite oxide thin film formed using a (110) oriented substrate. According to this disclosure, the formed thin film allows shear deformation of the crystal lattice during switching when the in-plane fourfold symmetry of the (110) oriented substrate is broken. That is, in a thin film formed in accordance with Patent Document 4 the crystal lattice is oriented parallel to the substrate plane, while the charge-ordered plane or orbital-ordered plane is non-parallel to the substrate plane. As a result, first order phase transitions involving deformation of the crystal lattice are possible even with a single crystal thin film in which the in-plane crystal lattice is fixed to the in-plane lattice of the substrate. Thus, according to Patent Document 4, a transition or in other words a switching phenomenon at high temperatures equivalent to those obtained with the bulk single crystal can be achieved by using a (110) oriented substrate.
Patent Document 5 (Japanese Patent Application Publication No. 2008-156188) also discloses an example of such an A-site ordered perovskite manganese oxide, formed as a thin film. According to Patent Document 5, an amorphous thin film was formed by a photo-assisted deposition process, and then laser annealed to achieve crystallization and A-site ordering. Specifically, A-site ordering of a SmBaMn2O6 thin film formed on a (100) oriented SrTiO3 substrate (lattice constant 0.3905 nm) was confirmed by electron beam diffraction.
However, the switching phenomena are suppressed in a single-crystal thin film of perovskite manganese oxide formed on a (100) oriented substrate. As a result, even if a substance or material exhibiting a charge-ordered phase within a temperature range suited to practical use (such as room temperature) can be prepared using single crystal bulk, it cannot immediately be applied to a device. Patent Document 5 does not disclose whether or not the thin film subjected to A-site ordering is a single-crystal thin film, but supposing it to be a polycrystalline film, or in other words a film comprising multiple grains with different crystal orientations on the same substrate, A-site ordering and charge and orbital ordering would then be impeded by lattice defects in the thin film. Thus, in the substance formed as a thin film in Patent Document 5 there is a concern of a decreased transition temperature or even the loss of the first order phase transition itself in extreme cases.
As in an ordinary semiconductor device, a single-crystal thin film must be prepared with few defects in order to achieve high-performance switching properties and uniform properties with a perovskite manganese oxide. One possible way of doing this is by using a (110) oriented substrate as disclosed in Patent Document 4 and the like. In a (110) oriented thin film formed using a (110) oriented substrate, the atomic stacking planes are arranged as (Ln,Ba)BO—O2—(Ln,Ba)BO. This describes a stacked body of atomic layers with a repeating structure obtained by forming one atomic layer consisting of A sites containing Ba atoms or a rare earth element Ln in an random pattern, B sites, and O atoms, and then forming an atomic layer containing two O atoms adjacent to this atomic layer. Thus, A-site ordering in a (110) oriented thin film must be in a plane parallel to the atomic stacking plane. However, some factor must provide a driving force for ordering the A-sites within the plane. In fact no such factor exists, and ordering the A sites of a (110) oriented thin film is not an easy matter.
One solution would be to use a (210) oriented substrate to form a (210) oriented perovskite manganese oxide film with a stacking structure of atomic stacking planes arranged in an AO-BO2-AO . . . configuration. This is because A-site ordering is easy in this stacked body of atomic planes, and the in-plane symmetry is also broken.
However, in a perovskite manganese oxide thin film grown on a (210) oriented substrate so that the atomic planes are arranged as AeO-BO2-LnO-BO2 . . . in the direction perpendicular to the substrate plane or in other words in the direction of [210] axis, the charge- and orbital-ordered plane is inclined at a large angle to the substrate plane in the formed crystal lattice as discussed below with reference to FIGS. 7 and 8. Therefore, the inventor of this application realized that if the aim is to use the resistance change resulting from an insulator-metal transition or other switching phenomenon, the usable resistance change is not sufficient in such a perovskite manganese oxide thin film.
That is, in a perovskite manganese oxide thin film grown on a (210) oriented substrate in a direction perpendicular to the substrate plane, the angle of the charge- and orbital-ordered plane 11 (FIG. 8) relative to the substrate plane exceeds 45 degrees. Specifically, the angle θ1 of the charge- and orbital-ordered plane or in other words the (010) plane relative to the (210) plane (substrate plane) is given by substituting m=2 in the following formula:θ1=arccos(1/(1+m2)1/2)  Formula 1,resulting in a value of about 63.4 degrees for θ1. Therefore, if the aim is to use the change in electrical properties caused by the insulator-metal transition as a change in electrical resistance in the direction of film thickness, the charge- and orbital-ordered plane 11 becomes a current pathway because it is formed with an aspect close to the direction of flow of the carrier. That is, the problem is that with this aspect of the charge- and orbital-ordered plane the resistance change generated by the insulator-metal transition is reduced, and the usable resistance change may be too small when the perovskite manganese oxide thin film is used as a device.